Make use of Lucene's compound format configurable

[jan-niestadt]

By default, Lucene will merge segment files into a single compound file to use fewer file descriptors. This takes a bit of time (7-33% during indexing according to docs) and is not necessary if your ulimit is set high enough. During development it's also easier to be able to see individual segment files.

We can add an option to IndexTool and BLS so users can choose to disable the compound format.